1. Field
The following description relates to a satellite communication system, and more particularly, to a multi-beam based transmission apparatus and method using multi-carrier.
2. Description of the Related Art
As a satellite communication system in use of complementary terrestrial components (CTC), such as repeaters, complementary ground components (CGC), and ancillary terrestrial components (ATC), currently a satellite digital multimedia broadcasting (DMB) service is being provided in South Korea. In European countries, since around 2010 researches on Digital Video Broadcasting-Satellite services to Handhelds (DVB-SH) systems have been actively carried out. In addition, some of U.S. companies, such as MSV and Terrestar, have developed an integrated satellite-terrestrial system to provide voice and data communications in urban and rural areas using ATC.
The DMB system in South Korea has been designed to use a terrestrial network that uses the same channel gap filler with a satellite to thereby enable vehicles and fixed and mobile terminals to receive high-quality audio signals and multimedia signals. The DMB system is optimized to a frequency bandwidth in the range of 2630 to 2655 MHz of both satellite and terrestrial parts. The DMB system includes a feeder link earth station, a broadcasting satellite, two types of terrestrial repeaters, and a receiver, for example, a receiver for a vehicle, a fixed terminal, or a mobile terminal. At this time, for an uplink, a band (e.g., 14 GHz) for a fixed satellite service (FSS) is used. The satellite converts the received signal into a 2.6 GHz band signal, and the converted signal is amplified to have a predetermined magnitude by an amplifier in the repeater of the satellite and is broadcast to a terminal that is located in a service area.
It is required for the terminal to receive a signal transmitted from the satellite through a small antenna having low directivity. For this purpose, the terminal needs to have sufficient effective isotropic radiated power (EIRP). Thus, the satellite needs to have a large transmitting antenna and a high-power repeater. When the satellite transmits a 2.6 GHz band signal, a shadow problem occurs due to obstacles on a path from the satellite. In order to overcome this problem, at the time of designing a system, it is required to additionally provide a repeater that retransmits a satellite signal. The repeater allows the signal transmitted from the satellite to be transmitted to places where the signal cannot reach due to band obstacles, such as buildings. The repeater is divided into a direct amplification repeater and a frequency conversion repeater.
The direct amplification repeater only amplifies a 2.6 GHz band signal that is received from the satellite. The direct amplification repeater uses a low gain amplifier to prevent an unnecessary divergence from occurring due to signal interference between a receiving antenna and a transmitting antenna. The direct amplifier covers a small area at a distance of 500 m from the repeater on the basis of a line of sight (LoS). Meanwhile, the frequency conversion repeater covers a large area at a distance of 3 km from the repeater, and converts a 2.6 GHz band signal transmitted from the satellite into another frequency band (e.g., 11 GHz) signal and transmits the converted signal to the terminal. In an environment where the two types of repeaters are needed, multipath fading occurs when two or more signals are received by the terminal, and a rake receiver with CDM technology adopted is used to stably receive multipath fading signals.
As another example of the mobile satellite communication system, the DVB-SH system provides services to a terminal using a satellite for nationwide coverage, and services to the terminal using the CGC for an indoor environment and terrestrial coverage. The DVB-SH system provides a mobile TV service at a 15 MHz bandwidth of an S band on the basis of a DVB-H. In to this case, the DVB-SH system uses a band near to a band used for terrestrial international mobile telecommunication (IMT) of an S band. Accordingly, integration with the terrestrial IMT and network reuse with a terrestrial system is easy, which results in decreasing installation costs.
The DVB-SH system in European countries considers a hybrid broadcasting structure with the terrestrial system. In order to resolve a signal interference problem between the satellite and the CGC and efficiently use a frequency, the DVB-SH system considers a structure in which a reuse factor is set to 1 with respect to a CGC cell in one satellite spot beam and to 3 with respect to the satellite spot beam.
For example, in France, by using the satellite spot beam, 9 TV channels can be broadcast in nationwide coverage, and 27 channels can be broadcast through the terrestrial repeater in a downtown area or an indoor environment.
Finally, a geostationary orbit (GEO)-based mobile satellite communication system has been developed in Mobile Satellite Ventures (MSV) and Terrestar in U.S. in order to provide to a PCS/cellular-type terminal a ubiquitous wireless broadband communication service such as an Internet access service and a voice conversation service in L and S bands. The GEO-based mobile satellite communication system uses a hybrid wireless network structure where the satellite and the ATC are coupled to each other and provides voice or high-speed packet services through the ATC, that is, a terrestrial system in a downtown area or congested area, and services through the satellite in the country or areas outside the downtown that are not covered by the ATC in U.S. or Canada. Since the ATC uses a wireless interface such as the satellite, the GEO-based mobile satellite communication system has been developed such that satellite services can be provided without increasing complexity of the terminal.
All personal portable mobile satellite communication systems that will be developed use a satellite in the country or areas outside the downtown where a line of sight is secured are scheduled to provide services using a complementary terrestrial component, and using the complementary terrestrial component in the downtown area or an indoor environment where satellite signals are not secured.
Recently, orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) technologies have been researched as an effective method of is high-speed data transmission over wired/wireless channels in the 4th generation terrestrial mobile communication system. The OFDM technology uses multi-carriers to transmit data. In the OFDM technology, a serially input symbol row is converted into a parallel data streams. Then, each of the parallel data streams is modulated with multiple orthogonal sub-carriers, that is, with multiple sub-carrier channels, and then transmitted. OFDM is similar to conventional frequency-division multiplexing. However, OFDM is characterized in transmitting data while maintaining orthogonality between multiple sub-carriers, and thereby achieving optimal transfer efficiency in high-speed data transmission. In addition, OFDM has enhanced frequency utilization efficiency and is robust against multipath fading, and thus can realize optimal transfer efficiency in high-speed data transmission. Furthermore, since frequency spectrum can be used in an overlapping manner, more efficient frequency utilization can be achieved and OFDM can be robust against frequency-selective fading and multipath fading, as well as impulsive noise. Also, it is possible to reduce interference between symbols using a guard interval, and to achieve an equalizer with a simple hardware design. Due to the above characteristics, OFDM has been actively utilized in a terrestrial communication system. OFDMA is an OFDM-based multiple access scheme. OFDMA enables a plurality of users, that is, a plurality of terminals, to divide and use sub-carriers in one OFDM symbol, and divides frequency and time and allocates the divided frequency and time to each user.
OFDMA is advantageous in that it can provide frequency and time in various ways according to user's demand. Thus, OFDMA can provide a variety of quality of service. In addition, OFDMA adaptively allocates sub-channels of OFDM according to user's channel environment, and thus can maximize capacity and can be used simultaneously with TDMA.
To reduce chipset cost of a terminal, it is important for a satellite interface and a terrestrial interface to have commonality. However, long propagation delay and large spot beam coverage which are characteristics of the satellite require a wireless interface on the earth to be modified for the satellite to reuse. Thus, a technology for overcoming long propagation delay and a method for reusing a frequency to maximize frequency efficiency have been introduced to apply OFDM or OFDMA in the satellite environment.
However, the suggested technologies and methods are characterized in that, unlike a terrestrial system, a satellite channel of LOS does not experience multipath fading and channels for different users have similar characteristics. Hence, the satellite environment cannot fully utilize the advantages of OFDM or OFDMA scheme of the terrestrial system.